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.A normal (Spanning Tree) mode port can take up to 20seconds to transition from Blocking to Listening, plus 15 seconds from Listening toLearning, plus another 15 seconds from Learning to Forwarding state.A Catalyst 5000 switch has one Spanning-Tree bridge ID per VLAN.If a link layer loopexists in a network topology, it may affect multiple Spanning Trees.You may lower thenumeric value of the bridge priority of a Catalyst switch in order to increase its chance ofbecoming the root.If you want to distribute the load on parallel VLAN InterSwitch Links,change the priority of each of the ports for the appropriate VLAN.The effect of this actionis that traffic from some VLANs will travel over one trunk, while traffic from other VLANswill travel over the other trunk.This is clearly a better result than having all traffic travelthrough one trunk while the other trunk stays in blocking mode.Troubleshooting Catalyst 5000 Trunking: VTP and ISLInterswitch trunk links carry traffic associated with different VLANs.There are two basicrequirements for a switch to accept and forward a frame from or to a trunk port.The firstrequirement is that the switch must recognize the VLAN that the frame is associated with.For instance, if a switch receives a frame associated with VLAN 5, but VLAN 5 has not yetbeen created on the switch, the switch will not process that frame.The second requirement is that both of the switches on a trunk (ISL link) must agree on theVLAN numbers that are allowed to traverse that trunk.For instance, if a switch has VLANs1, 2, and 3 allowed on a trunk, but the other switch connected to this trunk has only VLANs1 and 2 allowed, VLAN 3 will not be able to travel through this link.As you can imagine,without any automation for creating, deleting, and renaming VLANs and adding or deletingthem to or from trunk ports, there can be a substantial amount of manual configurationneeded on the part of the network administrator.This also means that there is a potential formisconfigurations, mismatches, loss of connectivity, and many other network problems.VTP addresses the concerns that were just expressed.VTP is a Layer 2 multicast messagingprotocol.This protocol allows switches that are put in a common administrative group,called a VTP domain, to communicate with each other, across the trunk links, regardingcreation, deletion, and renaming of VLANs.All VLANs are by default allowed to travelthrough a trunk port; however, those that are deemed unneeded can be removed from atrunk manually (referred to as manual pruning) or by using an automated process calledautomated pruning.It is crucial, however, to know that VLAN 1,which is the managementVLAN on Catalyst 5000 switches, cannot be pruned out of a trunk.VTP provides an automated means for having a consistent VLAN configuration throughouta VTP domain.A VTP domain comprises a group of interconnected switches, all of which 250 Chapter 8: Diagnosing and Correcting Catalyst Problemsare configured with a common VTP domain name.Since VTP messages travel throughtrunk ports, each switch must be connected to the rest of the VTP domain members via atleast one trunking link.A switch can be associated with only one VTP domain at a time.VTP uses a reserved VLAN (usually VLAN 1) for sending its advertisements, calledadverts (according to Cisco documentation, the VLAN number depends on the media typeused by the trunk).The VLAN number associated with VTP cannot be changed or deleted.For troubleshooting purposes, remember that VTP relies on the existence of trunks andVLAN 1 (the VLAN that carries VTP information).Also, make a note that CWSI relies on VTP and that Vlan Director s operation requires atleast one VTP server.A switch can be in one of server, client, or transparent modes with respect to the role it takeswithin a VTP domain.By default, a switch is in server mode; this means that theadministrator of this switch can create and delete VLANs on this switch using CLI(command-line interface), SNMP, or appropriate network management tools.A servergenerates, accepts (and implements), and forwards VTP adverts.Since a VTP server keepsa copy of VTP information in its NVRAM, it does not lose that information when it isreloaded.On a switch that is in VTP client mode, VLANs cannot be deleted and created.However, clients accept (and implement) VTP adverts received from other switches (thatare members of the same VTP domain), and they also forward the information to otherswitches.Clients do not keep the VTP information in their NVRAM; hence, upon reloadthey must receive the VTP information from other switches.You can probably guess why you will need at least one and preferably two VTP serverswithin a VTP domain.Switches that are members of a domain, but are in transparent mode,receive and forward VTP adverts but do not implement them.In other words, VLANs canbe and must be created, deleted, and renamed manually using CLI, for instance on aswitch in transparent mode.Hence, the consistency of a transparent mode switch s VTPinformation remains the responsibility of its administrator.If you create, delete, or renameVLANs on a transparent mode switch, the switch will not advertise that information to theother switches that are the members of its VTP domain.As mentioned earlier, though, if thisswitch (transparent mode) receives an advert, it will forward it, without implementing it.The VTP information pertaining to a particular VTP domain name has a revision numberassociated with it.Every time a change (VTP eligible) is implemented in a VTP domain,the revision number is incremented.In a working, interconnected, and consistent VTPdomain, the VTP revision number will be the same on all the (non-transparent mode)switches that are members of that VTP domain.Of course, when a change is implementedon a server, the revision number on the other switches will be one behind, due to the factthat the change has incremented the revision number on the server by one
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